Lock接口
与synchronized关键字相比拥有了锁获取与释放的可操作性，可非阻塞的获取锁、可中断的获取锁、超时获取锁。
标准接口定义
void lock();
void lockInterruptibly() throws InterruptedException;//可中断的阻塞获取锁
Condition newCondition();
boolean tryLock();//非阻塞的获取锁，马上返回
boolean tryLock(long time, TimeUnit unit) throws InterruptedException;//最大阻塞time时间，超时之后返回，可中断
void unlock();//释放锁
标准使用方式
注意不要将获取锁的过程写到try块里面
Lock lock = new ReentrantLock();
lock.lock();
try{
//to do
}finally{
lock.unlock();
}
队列同步器AQS（AbstractQueuedSynchronizer）
简单的理解是对于请求获取锁的线程，如果当前锁已经被别的线程获取，那么当前线程需要到队列中排队。如果队列中的第一个线程释放了锁，那么就会唤醒排在它后面的一个线程去获取锁（如果非公平锁，可能会被插队）。
核心字段
private volatile int state;//同步状态
private transient volatile Node head;//同步队列的头指针
private transient volatile Node tail;//同步队列的尾指针
核心方法
int getState();
void setState(int newState);
boolean compareAndSetState(int expect, int update);
boolean isHeldExclusively()；
void acquire(long arg);
boolean release(long arg);
void acquireShared(long arg);
boolean releaseShared(long arg);
同步队列的Node节点
这里最好结合源码的英文注释理解
static final class Node {
/** Marker to indicate a node is waiting in shared mode */
static final Node SHARED = new Node();
/** Marker to indicate a node is waiting in exclusive mode */
static final Node EXCLUSIVE = null;
/** waitStatus value to indicate thread has cancelled */
static final int CANCELLED = 1;
/** waitStatus value to indicate successor's thread needs unparking */
static final int SIGNAL = -1;
/** waitStatus value to indicate thread is waiting on condition */
static final int CONDITION = -2;
/**
* waitStatus value to indicate the next acquireShared should
* unconditionally propagate
*/
static final int PROPAGATE = -3;
volatile int waitStatus;
volatile Node prev;
volatile Node next;
volatile Thread thread;
Node nextWaiter;
final boolean isShared() {
return nextWaiter == SHARED;
}
独占式同步状态的获取源码分析
acquire方法
首先非阻塞的获取同步状态（tryAcquire方法），如果失败的话会把当前线程信息构造一个Node节点加入到同步队列的尾部进行排队（这里可能会使线程进入waiting状态）
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
tryAcquire方法
非阻塞的获取同步状态，失败返回false.这个是子类实现的。（模板方法模式）
addWaiter方法
将当前节点加入到同步队列的尾部
private Node addWaiter(Node mode) {
// 这里mode为EXCLUSIVE,也就是 node.nextWaiter==null.
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
Node pred = tail;
//下面if逻辑是尝试快速插入到队列尾部，如果失败到enq函数
if (pred != null) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
enq(node);//当前节点插入到队列尾部
return node;
}
enq 方法
当前节点插入到队列尾部，保证成功，可能重试多次。采用CAS更改改共享变量的标准写法。
private Node enq(final Node node) {
for (;;) {
Node t = tail;
if (t == null) { // Must initialize
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
}
acquireQueued方法
对于刚进入队列排队的节点或者刚被唤醒的线程（LockSupport.unpark(thread)），会检查它前面的那个节点是不是head节点，如果是的话尝试获取锁。如果不是的话会判断当前线程是否可以到waiting状态(释放cpu资源，避免盲等)，（判断的依据主要是当前节点的前一个节点的waitStatus是否为SIGNAL），如果可以睡眠，就会睡眠直到被前一个节点唤醒。
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
//如果节点的前驱节点为头节点，并且获取同步状态成功，则把当前节点设置为头节点，然后返回
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
// 判断当前节点是否可以睡眠或者把当前节点状态改到可睡眠状态（可以睡否最重要的依据是睡后有人唤醒你，否则你就醒不来了）
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
shouldParkAfterFailedAcquire方法
如果暂时不能获取同步状态，线程会考虑睡一会（park）
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
//如果当前节点的前驱节点的waitStatus为Node.SIGNAL，则其前驱节点在释放锁之后就会唤醒（uppark）当前线程,所以当前线程可以放心的park
if (ws == Node.SIGNAL)
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
//如果前驱节点已经放弃获取锁，更新其前驱节点
if (ws > 0) {
/*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {
/*
* waitStatus must be 0 or PROPAGATE. Indicate that we
* need a signal, but don't park yet. Caller will need to
* retry to make sure it cannot acquire before parking.
*/
//尝试把前驱节点状态改为Node.SIGNAL
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
parkAndCheckInterrupt方法
当前线程park,到waiting 状态，线程阻塞于当前方法直到被中断或者unpark
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
独占式同步状态释放源码分析
release方法
释放同步状态，唤醒队里中的下一个节点。
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
unparkSuccessor方法
唤醒队列中下一个节点去参与锁的竞争
private void unparkSuccessor(Node node) {
/*
* If status is negative (i.e., possibly needing signal) try
* to clear in anticipation of signalling. It is OK if this
* fails or if status is changed by waiting thread.
*/
int ws = node.waitStatus;
if (ws < 0)
compareAndSetWaitStatus(node, ws, 0);
/*
* Thread to unpark is held in successor, which is normally
* just the next node. But if cancelled or apparently null,
* traverse backwards from tail to find the actual
* non-cancelled successor.
*/
Node s = node.next;
if (s == null || s.waitStatus > 0) {
s = null;
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
LockSupport.unpark(s.thread);
}
共享式获取同步状态源码分析
acquireShared方法
如果没有获取同步状态成功，就会将当前节点更新加入到同步队列。
public final void acquireShared(int arg) {
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}
doAcquireShared
同步队列中获取独占式同步状态，类似于独占模式同步状态的获取。
private void doAcquireShared(int arg) {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
//如果当前节点获取同步状态成功，会将当前节点设为头节点，
//并且传播到它的下一个节点（下一个节点尝试获取同步状态）
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
setHeadAndPropagate方法
设置node节点为head节点，并且尝试传播到下一个节点(因为共享模式的同步状态是可以有多个线程同时获取的)
private void setHeadAndPropagate(Node node, int propagate) {
Node h = head; // Record old head for check below
setHead(node);
if (propagate > 0 || h == null || h.waitStatus < 0 ||
(h = head) == null || h.waitStatus < 0) {
Node s = node.next;
//如果下一个节点是共享模式的节点，就考虑唤醒下一个节点去竞争性获取共享状态。
if (s == null || s.isShared())
doReleaseShared();
}
}
共享式释放同步状态源码分析
releaseShared方法
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
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